Effect of Large Lattice Mismatch on Superconductivity and Plasmonic Properties of Epitaxial NbTiN Films

Uncovering the effect of large lattice mismatch on epitaxial growth, microstructures, and electrical and plasmonic properties of rock-salt transition-metal nitride (TMN) thin films is fundamentally important to their epitaxial integration with many functional oxide materials for various applications including superconducting quantum and nanophotonic devices. However, the study of a large lattice mismatch effect on the structural, electrical, and plasmonic properties of TMN films remains unclear. Here, we have grown rock-salt niobium titanium nitride (Nb_0.5Ti_0.5N) films on small lattice-mismatched MgO (∼−2.47%) and large lattice-mismatched YAlO₃ substrates (∼−17.46%), respectively, by reactive sputtering. It is remarkable that Nb_0.5Ti_0.5N/YAlO₃ films are still single-crystalline and have superior superconducting and surface-enhanced Raman scattering (SERS) performance over Nb_0.5Ti_0.5N/MgO epitaxial films. It is indicated that the superconductivity in Nb_0.5Ti_0.5N/YAlO₃ films is less suppressed by in-plane compressive strain compared to that in Nb_0.5Ti_0.5N/MgO films. The roughened surface morphology on Nb_0.5Ti_0.5N/YAlO₃ films contributes to the enhanced SERS performance, in good agreement with the simulation results from the finite-difference time domain study. Our study can be beneficial to the epitaxial integration of rock-salt TMN films with functional oxides of a wide spectrum of lattice mismatches with enhanced physical properties including superconducting and SERS performance.